In recent years, detection of gas in the environment has become increasingly important for avoidance of danger caused by noxious gas, odorless carbon monoxide gas and the like in the environment. As a gas sensor for detection of such gas in the environment, semiconductor type gas sensors are conventionally known (e.g., patent document 1). This gas sensor is a P-N type gas sensor that electrically detects resistance value that changes when N-type semiconductor and P-type semiconductor made from metal oxide and in contact with each other are maintained at a high temperature and a gas in the environment comes into contact with the contact part of the both semiconductors, where the detection utilizes change of resistance value of the sensor. Recently, a P-N type carbon monoxide gas sensor capable of detecting carbon monoxide gas with high selectivity has also been proposed (patent document 2). In recent times, considerations have been made of a gas sensor using a single-walled carbon nanotube (SWCNT) (non-patent document 1, patent document 3), and a gas sensor using a stannous oxide (SnO2) nanowire (non-patent document 2).
P-N type gas sensors can simultaneously detect, besides carbon monoxide gas, organic gases such as methane, ethanol, ethyl acetate and the like. In addition, P-N type carbon monoxide gas sensor can detect carbon monoxide gas alone. However, when used singly, such gas sensors cannot distinguish the kind of gas with good sensitivity. It lacks stability at room temperature, requires detection at a high temperature using a heater, and consumes a large amount of electric power. Moreover, since bulk crystal is used for a metal oxide semiconductor, sufficient detection sensitivity can be obtained only when a gas detector part (laminate of N-type semiconductor device and P-type semiconductor device) has a certain large size, which prevents sufficient downsizing of the sensor. On the other hand, when a gas detector part has a large size to improve sensitivity, gas detection responsiveness becomes low. Since it requires a P-type semiconductor and an N-type semiconductor, the cost becomes high and the production steps become complicated.
A gas sensor using a single-walled carbon nanotube (SWCNT) contains the single-walled carbon nanotube (SWCNT) as a principal member, and production thereof requires a large-scale, complicated manufacturing equipment, thus posing problems in terms of cost and mass productivity. In addition, the gas to be the detection target is inorganic gas such as hydrogen gas, helium gas, argon gas and the like, dioxide nitrogen and the like, and the sensor is unsuitable for the detection and distinction of organic gas. Also, a gas sensor using a tin oxide (SnO2) nanowire has a high operation temperature and requires heating, which leads to a high consumption power.
In recent years, building materials containing many noxious volatile organic compounds (formaldehyde, acetaldehyde, toluene, trichloroethylene, tetrachloroethylene, benzene, xylene, acetone, methanol and the like) such as paint, adhesive, waterproof material and the like are used in construction sites such as new construction of office building, apartment building and the like, repair work of outer wall etc., and the like. Emission of volatile organic compounds from such building materials into the surrounding environment has been recognized as one of the factors causing sick-house syndrome, and therefore, improvement of the detection technique of organic gas in the environment is important. Moreover, since such volatile organic compounds release explosive organic gas, improvement of the detection technique for avoidance of danger is important. Furthermore, the significance of an ethyl alcohol detection technique as a countermeasure against drunken driving is also increasing.